Small heat rise may offer big boost for malaria

Even small changes in temperature may contribute to the spread of malaria in the East African highlands, a new modelling study has found - a result sharply contrasting with previous research.

The latest work suggests populations of malarial mosquitoes could grow substantially with just a small rise in temperatures. For example, the mathematical models suggest a 3% rise in local temperature from one year to the next can mean a 30% to 40% increase in mosquito abundance. Experts note the new research also uses five more years of temperature data from Africa that previous work did not.

Up to 2.7 million people die each year from malaria, and the majority of them are African children. Because areas of higher altitude are cooler, the mosquitoes that carry malaria have a tougher time reproducing there and spreading the illness. "In the highlands, mosquito abundance is typically very low," says ecologist Mercedes Pascual of the University of Michigan at Ann Arbor in the US.

But she says health officials have seen a rising number of malaria cases in these highland regions. In one dramatic example, the tea estates of Kericho in western Kenya saw a rise in severe malaria cases from 16 per 1000 people in 1986 to 120 per 1000 people in 1998.

El Niño effect

In 2002, other researchers failed to find a connection between climate change and the growth of malaria in the highlands of east Africa (Nature, vol 415, p 905).

Yet some studies have linked large epidemics of malaria with climate and temperature anomalies in other regions, such as in the Indian subcontinent and Uganda. And earlier in 2006 scientists showed how El Niño-related climate variability can be linked to malaria outbreaks in Botswana (Nature, vol 415, p 576).

A combination of modified algorithms and additional data gave Pascual and her colleagues a new view on how climate might influence malaria in African highlands. Their work relied partly on information from laboratory experiments into how mosquitoes respond to different temperatures.

Drug resistance

The group documented a warming trend of about 0.5°C in Africa's highland regions from 1970. Even this, they calculate, could have had a profound effect on the number of mosquitoes able to breed in higher-altitude areas.

"But it doesn't prove that this is the main or only driver," Pascual concedes. Other factors, such as changes in land use and growing resistance to anti-malaria drugs also influence the spread of the disease.

The researchers have yet to relate their models of climate and mosquito populations to data gathered about human cases of malaria in Africa over the past half century. "It would be an interesting and important step," Pascual says.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0508929103)

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